The magnetic random access memory (MRAM) is expected to serve as a nonvolatile memory that can operate at high speed and is rewritable an infinite number of times, and therefore intensively developed. In the MRAM, a magnetic material is used as a memory element, and information is memorized corresponding to a direction of magnetization of the magnetic material. As a method for switching the magnetization of the magnetic material, there are proposed several methods; however, they are common in that any of them uses electric current. To put the MRAM into practical use, reducing a write current is very important. For example, according to N. Sakimura, et al., “MRAM Cell Technology for Over 500 MHz SoC”, 2006 Symposium On VLSI Circuits, Digest of Technical Papers, pp. 136, 2006, the write current is required to be reduced to 0.5 mA or less, and more preferably to 0.2 mA or less.
Among methods for writing information to the MRAM, the most popular one is a method that switches a magnetization direction of a magnetic memory element with a magnetic field generated by current flowing through a write line arranged around the magnetic memory element. This method enables writing at 1 nanosecond or less in principle because of magnetization switching by the magnetic field, and is therefore preferable to achieve a high speed MRAM. However, a magnetic field for switching magnetization of a magnetic material that ensures thermal stability and disturbance magnetic field resistance is typically around a few 10s Oe (oersted). To generate such a magnetic field, a current of around a few mA is required. In such a case, a chip area is inevitably increased, and a power consumption required for writing is also increased. For this reason, such MRAM is less competitiveness as compared with the other random access memories. In addition to this, as the element is miniaturized, the write current further increases, which is not preferable also from the perspective of scaling.
In recent years, as means adapted to solve such problems, the following two methods are proposed. A first method is a method using spin injection magnetization switching. This is a method that, in a stacked film including a first magnetic layer having reversible magnetization and a second magnetic layer electrically connected to the first magnetic layer and having pinned magnetization, when the current flows between the first and second magnetic layers, uses an interaction between spin-polarized conduction electrons and localized electrons in the first magnetic layer to switch the magnetization of the first magnetic layer. The spin injection magnetization switching occurs at a certain current density or more. Therefore, as an element size is decreased, a current required for writing is reduced. That is, it can be said that the spin injection magnetization switching method is superior in the scaling property. However, in general, an insulating layer is provided between the first and second magnetic layers, and upon writing, a relatively large current should flow through the insulating layer, which causes a problem in write disturbance resistance or reliability. Also, since a write current path and a read current path are the same, erroneous writing upon reading is also concerned. As described, the spin injection magnetization switching is superior in the scaling property, but has some barriers for practical application.
On the other hand, a second method is a magnetization switching method using a current-driven domain wall motion phenomenon. The magnetization switching method can solve the above-described problems associated with the spin injection magnetization switching. An MRAM using the current-driven domain wall motion phenomenon is disclosed in, for example, Japanese Patent Publication JP 2005-191032 A. In the MRAM using the current-driven domain wall phenomenon, magnetizations at both end parts of a first magnetization layer having reversible magnetization are pinned so as to be substantially antiparallel to each other. In such magnetization arrangement, a domain wall is introduced into the first magnetization layer.
Note that, as reported in A. Yamaguchi et al., “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, Physical Review Letters, vol. 92, No. 7, pp. 077205 (2004), if a current is made to flow in a direction in which the current passes through a domain wall, the domain wall moves toward a direction of conduction electrons. Based on this, by making the current flow through the first magnetic layer, it is possible to write information. The current-driven domain wall motion also occurs at a certain current density or more, and therefore it can be said that the current-driven domain wall motion has the scaling property similarly to the spin injection magnetization switching. In addition to this, in the MRAM using the current-driven domain wall motion phenomenon, a write current does not flow through an insulating layer, and a write current path and a read current path are separated. For this reason, the above problems as described in the spin injection magnetization switching can be solved.
As a related technique, Japanese Patent Publication JP 2005-150303 A discloses a magnetoresistance effect element and a magnetic memory. The magnetoresistance effect element includes a ferromagnetic tunnel junction including a three-layered structure of a first ferromagnetic layer/a tunnel barrier layer/a second ferromagnetic layer. The first ferromagnetic layer has a larger coercive force than the second ferromagnetic layer, and a tunnel conductance is varied depending on a relative angle between magnetizations of the two ferromagnetic layers. In the magnetoresistance effect element, magnetization of an end part of the second ferromagnetic layer is pinned in a direction having a component orthogonal to an easy magnetization axis of the second ferromagnetic layer.
Japanese Patent Publication JP 2006-73930 A discloses a varying method of a magnetization state of a magnetoresistance effect element using domain wall motion, a magnetic memory element using the method, and a solid magnetic memory. The magnetic memory device includes a first magnetic layer, an interlayer, and a second magnetic layer. Information is recorded on the basis of magnetization directions of the first and second magnetic layers. The magnetic memory element regularly forms: in at least one of the magnetic layers, domains of which magnetizations are antiparallel to each other; and a domain wall that separates between the domains. Then, by moving the domain wall in the magnetic layer, locations of the adjacent domains are controlled to record the information.
Japanese Patent Publication JP 2006-270069 A (WO2006090626) discloses a magnetoresistance effect element and a high-speed magnetic recording device based on domain wall motion by a pulse current. The magnetoresistance effect element includes a first magnetization pinned layer/a magnetization free layer/a second magnetization pinned layer. The magnetoresistance effect element is provided with a mechanism for inducing a domain wall in a transition region between the magnetization pinned layer and the magnetization free layer, which serves as at least one of boundaries between the first magnetization pinned layer and the magnetization free layer and between the magnetization free layer and the second magnetization pinned layer. Also, directions of magnetizations of the magnetization pinned layers are set to be antiparallel to each other to make a structure having the domain wall in any one of the transition regions between the magnetization pinned layers and the magnetization free layer. By applying a current having a predetermined pulse width, the domain wall moves between the two transition regions with the current not exceeding a DC current density of 106 A/cm2, and thereby magnetization of the magnetization free layer is switched to detect a magnetoresistance associated with a variation in direction of relative magnetization.
However, the inventor has found the following problems. In the MRAM using the current-driven domain wall motion, there is a problem that an absolute value of a write current becomes relatively large. Besides the above Physical Review Letters, vol. 92, a large number of reports on the observation of current-induced domain wall motion are made, in which a current density of approximately 1×108 [A/cm2] is required for domain wall motion. In such a case, if a width and a film thickness of a layer in which the domain wall motion occurs are respectively assumed to be 100 nm and 10 nm, the write current is 1 mA. To reduce the write current down to this value or less, it is only necessary to decrease the film thickness. However, it is reported that as the film thickness is decreased, the current density necessary for writing is further increased (see, for example, A. Yamaguchi, et al., “Reduction of Threshold Current Density for Current-Driven Domain Wall Motion using Shape Control”, Japanese Journal of Applied Physics, vol. 45, No. 5A, pp. 3850-3853, (2006)). Also, reducing a narrow line width to a few 10s nm or less involves a large difficulty in fabrication technique. Further, in the case of using the current density close to 1×108 [A/cm2] for writing, influences of electron migration and temperature rise are concerned.
In addition to this, in the MRAMs using the current-driven domain wall motion, including the MRAM disclosed in JP 2005-191032 A, a current is used to move the domain wall between two pinning sites. In this case, the following write error is concerned. That is, the concerned case is the case where the domain wall starting from a pinning site serving as a starting point does not reach a pinning site serving as a reaching point, the case where the domain wall starting from the pinning site serving as the starting point moves beyond the pinning site serving as the reaching point, the case where the domain wall starting from the pinning site serving as the starting point reaches the pinning site serving as the reaching point, but when the write current is switched off, the reached domain wall further moves to the other stable point, or the like.